Bar end shifters work in the same way as down tube shifters, but only the location is different. However, during the approx. 5 years WITH indexed shifting but WITHOUT STI-like brifters, the use of indexed bar end shifters (or: bar-cons) was minimal. There were some reasons. The 1st is technical: for indexed shifting you need more than a shifting lever that says "click". The whole system has to be perfect. It took conservative cyclists and bicycle dealers a long time to understand that this was not only a commercial trick from Shimano. Shifting levers, sprockets, chains, cables, yes, even frame design has to be optimized. Then we come to the next point: 'cross riders tend to mix and match everything they can lay their hands on, which is not good for indexed shifting. And third, cross bikes are more complex than road bikes. Different gear ratios, longer cables, different cable routing, mud, rain, frost, etc.
A short explanation why indexed shifting in general and in the beginning STI was not perfect for CX.
Sun Tour had the perfect bar end shifters in the olden days of friction shifting. Operation was the same as Shimano, Campagnolo and others, but Sun Tour was extra popular because of the perfect Power Ratchet mechanism. A friction type shifting lever has to be set up with a certain amount of friction in order to withstand the spring load of the derailleur. The combination of lever friction, cable friction and derailleur spring force make shifting heavy and hard to control. Sun Tour Power Ratchet's ratchet and spring/pawl mechanism decouples the friction from the lever. When shifting against the derailleur's spring tension, the lever can be moved upwards without friction, making it light to operate. When a certain gear has been selected, the lever (and thus the derailleur) stays in place because of the lever friction and the ratchet mechanism. When shifting in the other direction (lever downwards), the rider has to overcome the lever friction, but gets help from the derailleur's spring force.
All in all the same idea as e.g. Simplex' and later Campagnolo successful down tube shifters with spring load, but a different construction.
Sun Tour bar end shifters were the overall standard for cyclocross in the 1980s and nearly every single 'cross bike was equipped with 1 or 2 of these levers, depending of the number of gears (single or double chain ring in the front). Nowadays these levers are pretty rare and of course getting more and more expensive. This week I found a set in Belgium, the cradle of cyclocross, and still for very good price. As you can see on the picture, on both levers the flat head nut is missing. This always happened with these levers. Most levers are without the nut. It's no big deal, the function remains okay and the levers don't fall apart. For security reasons I will put a standard M4 hexagon nut in place.
For (dis)assembly to (from) the handlebars, the lever has to be removed from the bracket. Just a matter of taking the long bolt and the nuts out and sliding the lever out of the bracket. When tightening the lever to the bar end, you have to turn the Allen key counter clockwise! Not because there's left hand threading, but because the tool is put into the bottom of the central fixing bolt instead of the bolt head. Turn clockwise in case you want to remove the lever from the bar end.
